Method and means for measuring the torque delivered by an electric motor

ABSTRACT

A torque feedback signal for the control system of a motorgenerator set is generated by a torque feedback circuit which includes a differentiator and a current sensing circuit. The differentiator is connected to sense generator output voltage and provide an acceleration torque signal which is proportional to the first derivative of generator voltage. The current sensing circuit provides an induced torque signal proportional to motor armature current, and this is summed with the acceleration torque signal to provide a feedback signal proportional to the torque at the motor rotor. A second embodiment is shown which separates the induced torque into steady-state and transient components. The transient component is summed with the acceleration torque signal to provide a transient torque feedback signal, and the steadystate induced torque component is fed back to establish a torque limit.

United States Patent 691 Stoner [54] METHOD AND-MEANS FOR MEASURINGTI-IE TORQUE DELIVERED BY AN ELECTRIC MOTOR [75] Inventor: Thomas A.Stoner, Brookfield, Wis.

[73] Assignee: Bucyrus-Erie Company, Milwaukee,

Wis.

[22] Filed: Feb. 15, 1973 [2!] Appl. No.: 332,697

52 us. c1.... .Q 318/432, 318/433 51 111601.... H02p 5/28 [58] Field orSearch 318/561, 466, 432, 433, 318/152 [56] References Cited UNITEDSTATES PATENTS 3,283,230 11/1966 Davies ct al. 318/561 X 3,565,4022/1971 Linke 318/466 X 3,631,326 12/1971 vil'kkala 318/466 X PrimaryExaminer-B. Dobeck Attorney, Agent, or FirmQuarles & Brady 11113,867,678 1451 Feb. 18, 1975 57 ABSTRACT A torque feedback signal forthecontrol system of a I motor-generator set is generated by a torquefeedback circuit which includes a differentiator and a current sensingcircuit. The differentiator is connected to sense generator outputvoltage and provide an acceleration torque signal which is proportionalto the first derivative of generator voltage. The current sensingcircuit provides an induced torque signal proportional to motor armaturecurrent, and this is summed with the acceleration torque signal toprovide a feedback signal proportional to the torque at the motor rotor.A second embodiment is shown which separates the induced torque intosteady-state and transient components. The transient component is summedwith the acceleration torque signal to provide a transient torquefeedback signal, and the steady-state induced torque component is fedback to establish a torque limit.

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II II TIME (SEC) METHOD AND" MEANS FOR MEASURING THE TORQUE DELIVERED BYAN ELECTRIC MOTOR BACKGROUND" or. THE INVENTION The general field of theinvention is methods and means for measuring the force delivered to aload by an energy conversion machine, and more specifically, to meansfor indirectly measuring the torque delivered by an electric motor to aload.

A d-c electric motor is an energy conversion machine which receiveselectrical energy at its armature circuit andconverts it to mechanicalenergy at its rotor by electromagnetic interaction of armature currentwith magnetic flux established by associated field windings. A torque isthereby induced into the rotor which causes it-and the attached load torotate. Commonly, when the motor is connected to a control system forgoverning.

rotation, rotor torque is sensed and a torque feedback signal isdeveloped which is used by the control system to vary the amount ofelectrical energy supplied to the motor. 7

It is a fundamental principle of both servo systems and regulatorcontrol systems that the magnitude of such torque feedback signals bedirectly proportional to the torque delivered by the motor. Twoapproaches are presently used to generate a torque feedback signal foran electric motor, neither of which is entirely satisfactory. The firstand most direct approach is to attach a sensing device, such as a straingauge, directly to an element of the load. Although such a directapproach may provide an accurateindication of motor torque, the sensingapparatus commonly used are often expensive to install and maintain,particularly in applications where the sensing device is subject tosevere environmental conditions.

The second, and the most common approach used to generate a torquefeedbackv signal is to electrically sense a motor operating parameterwhich indirectly indicates motor torque. With d-c motors, for example,the magnitude of the motors armature current is sensed and a torquefeedback signal proportional thereto is generated. If the do motor has acommutating winding it is also common to sense the voltage drop acrossthiswinding for an indication of torque. Similarly, with a-c motors-atorque feedback signal proportional to winding current squared can beused as an indication of the torque developed by the motor. Althoughprior indirect means of indicating motor torque are highly reliable andrelatively inexpensive to implement and to maintain, they are notentirely satisfactory for all control purposes.

In drive systems such as that disclosed in US. Pat. No. 3,518,444 issuedto D. E. Barber on June 30, 1970 and entitledControl System forExcavating Equipment, the magnitude of the torque generated by a d-c.hoist motor on an excavator is sensed by measuring the voltage generatedacross its commutating field winding. The torque feedback signal thusgenerated is proportional to the torque generated by the hoist motor. Ithas been discovered, however, that when such control systems operate asregulators to limit the torques and forces developed in the hoist drivemechanism of the excavator, the established torque limits are oftenexceeded during digging operations.

The excessive torques and forces which develop in excavators using priorcontrol systems are transient in nature, and occur primarily when thedipper strikes large objects such as rocks during-digging. Although theylast for a relatively short period of time, such excessive transienttorques and forces occur repeatedly and considerably shorten the usefullife of various elements in the drive system.

Considerable effort has been made to reduce these high transient torquesand forces which occur during digging and to thereby extend the usefullife of the excavator front end and particularly the useful life of thehoist rope. Such past efforts have been directed primarily to means ofshortening the response time of the control system. However, it hasbecome increasingly apparent that regardless of the response time of thecon trol system, these high transient torques and forces will continueto be generated as long as present torque sensing and measuringtechniques are used. Thus, there is a need for better torque sensing andmeasuring tech- .niques which will improve the response of controls totransient load conditions. Although such an improved system would findimmediate application in the hoist motor drives of excavators, a controlsystem providing such improved response would find application in avariety of drives.

SUMMARY OF THE INVENTION The present invention relates generally to animproved method of indirectly measuring the force delivered to amechanical system by an energy conversion machine. Morespecifically, itrelates to an indirect method and means of generating a torque feedbacksignal which is proportional to the torque at the rotor of an electricmotor. The invention stems from a discovery that the total load torqueat the rotor of an electric motor is equal to the algebraic sum of twocomponent torques; an induced torque component related to the energysupplied to the motor, and an acceleration torque component related tothe energy required to accelerate or decelerate the inertias of themotor rotor and attached drive system. The. invention includes the stepsof sensing the magnitude and sign of the acceleration torque componentand generating a signal proportional thereto, sensing the induced torquecomponent and generating a signal proportional thereto, and sumthingthese two signals to obtain a signal proportional .to the total loadtorque. The invention also includes a means of generating a torquefeedback signal for a motor, which means includes a first sensorconnected to sense motor armature current and generate an induced torquesignal proportional to armature current; and a second sensor including adifferentiator circuit connected to sense a voltage and generate inresponse thereto an acceleration torque signal proportional to the rateof change of the sensed voltage; and summing means connected to receiveand arithmetically add the induced torque signal and the accelerationtorque signal to obtain a rotor torque feedback signal.

It is a general object of the invention to provide an improved methodand means of indirectly measuring the torque applied to a mechanicalsystem by a motor. Prior torque measuring circuits for do motors, suchas that disclosed in the above cited US. Pat. No. 3,518,444, sensearmature current and therefore only indicate the magnitude of theinduced torque component. The resulting feedback signal is therefore notan accurate indication of the torqueactually present and applied at theload. The present invention, on the other hand senses the magnitude ofthe acceleration torque 3 component and adds this" quantity to theinduced torque component to provide an accurate indication of the totaltorque delivered to the load.

Another object of the invention is to provide a means of generating atorque feedback signal to a control system which more accuratelyreflects the torque actually being delivered to the load. In certainapplications, and in particular the hoist drive of large excavators, thecombined inertias of the mechanical system and the motor rotor generatelarge acceleration and deceleration torques during rapid speed changes.These acceleration-torques may alone exceed the torque limit which thecontrol system .is intended to establish.

Therefore, by sensing the acceleration torque component and combining itwith the sensed induced torque component, a torque feedback signal isgenerated which can more effectively control or regulate the torques andforces in the system. I

Anotherobject of the invention is to generate a torquefeedback signalfor a d-c motor. The induced torque component is indicated by sensingmotor armaturecurrent. The acceleration torque component is indicated bysensing the rate of change of the voltage applied to the motor armature.This is accomplished by a differentiator circuit. When the two signalsthus generated are summed, the resulting torque feedback signal containsthe information necessaryto effectively limit the torque applied to theload under all operating conditions. k I

Still another object of the invention is to provide an indirect torqueindicating means for the hoist drive control system on an excavator. Dueto the inherent time delays associated with large d-c motor drives, aparticularly useful transient-torque feedback signal generator using theteachings of the present invention has been developed. When used incombination with a conventional induced torque feedback circuit, a nettorque feedback signal is generated to the hoist drive control systemwhich providesa substantial reduction inthemagnitude of the transienttorques occurring in the system. j l

The foregoing andother objects. and advantages of the. invention willappear from the following description. In the description reference ismade to the accompanying drawings which form a part hereof, and in whichthere is shown by way of illustration preferred embodiments of theinvention. Such embodiments do not necessarily represent the full scopeof the invention, and reference is made to the claimsherein forinterpreting the breadth of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing ofa;motor drive systern,

FIG. 2'is an electrical schematicdi'agram of a motor drive controlsystemembodying the present invention, FIG. 3 is an electrical schematicdiagramof a motor I drive control system which includes an alternativeembodiment of the present invention,

FIGS. 4a, 4b, and 4c are graphs showing the operating characteristics ofprior hoist motor control systems,

FIG. 5 is a graph showing the operating characteristics of a hoist motorcontrol system which employs the circuit of FIG. 3

DESCRIPTION OF THE PREFERRED EMBODIMENTS As indicated, the generalobject of the present inven- 5 tion is to measure, or indicate the'forcedelivered to a load by an energy conversion machine. Referring to FIG.1, such an energy co nversion machine is shown as a motor 1. The rotorof the motor 1 is attached to drive a shaft 2 which connects through atransmission 3 to revolve a drum 4. The torque generated at the shaft 2v by the motor 1 is multiplied by the transmission 3, with anaccompanying reduction in rotary speed. One end of a rope 5 is attachedto the drum 4 and the other end extends over a sheave 6 to connect witha weight 7. The drum 4 acts to convert the torque generated by the motor1 to a linear force or tension on the rope 5 which in turn acts to raiseor lower the weight 7. It should be apparent to those skilled in the artthat the drive system schematically shown in FIG. 1 is representative ofany one of many drive systems in which an energy conversion machine isdriving an attached load. For example, it may represent the hoist drivesystem of an excavator, in which the rope 5 is a hoist rope, the sheave6 is a boom point sheave, and the item 7 is a dipper to which diggingforces are applied when the dipper is hoisted upwardthrough a bank ofearth.

When used as a hoist drive on an excavator, the generalized system ofFIG. 1 is attached to a control system 8, such as thatdisclosed in theabove cited patent-The control system 8 operates to control theelectrical energy supplied to the motor 1 and to thereby control therate of rotation of the shaft 2 and the hoist rate of the dipper 7. Thedigging forces acting on the dipper 7during hoisting cause abruptvariations in dipper velocity. These abrupt speed variations createinertia forces which act on the mechanical components'of the drivesystem and are reflected back through the drum 4 and transmission3.'Thefmotor 1 responds to the resulting variations in the torque at theshaft 2 to maintain the hoist rate demanded by the control system 8. Toprevent overloading, however, a torque limit is established, and whenthis limit is sensed by the-control system 8, motor speed is reduced anddigging proceeds at a slower rate until the obstruction generating thehigh digging force is cleared. Thus to prevent excessive forces frombeing developed in the drive system, and particularly in the rope 5, anaccurate indication of the torque delivered by the motor 1 to the shaft2 must be made and a corresponding torque feedback signal must begenerated to the control system 8.

Prior controlsystems generate a.torque feedback signal using indirectmeans of sensing torque at the shaft 2..The torque at the shaft 2 is anaccurate indication of thetorque applied to the gears in thetransmission 3, the torque applied to the drum 4, and the tension on therope 5 at least under relatively steady state coni m a where:

T, induced torque K motor torque constant l,,'= motor armature currentThe induced torque T, is that created by electromagnetic interactionwithin the motor 1, and is measured by sensing motor armature current.The induced torque is the total torque that would exist for a steadystate speed condition, and hence at times herein may be referred to as asteady state torque. Although this induced torque is equal to the torqueat the shaft 2 under steady state speed conditions, it is not equal to,or even proportional to the torque on the shaft 2 during transientconditions. Consequently, a control system subject to abrupt transientconditions cannot successfully limit torque by sensing a limit on motorarmature current.

It is a discovery of the present invention that the induced torqueindicated in the above expression represents only one component of thetotal load torque delivered by an electric motor. This component isrelated to the amount of electrical energy supplied to the motor 1 andconverted into mechanical energy. The above expression does not includethe torque component which results from the acceleration, ordeceleration of the inertias of the drive system. This latter torquecomponent is referred to herein as an acceleration torque. For example,assume that the weight 7 in FIG. 1 is hoisted at a constant rate andsuddenly encounters an immovable obstacle which brings it to a halt. Thetension in the hoist rope immediately rises, as does the torque at theshaft 2. In response to the excessive torque demand on the shaft 2,thecontrol system 8 decreases the energy supplied to the motor 1 in anattempt to maintain the. armature current and the induced torque at theshaft 2 under the preset limit.

Nevertheless, it has been found that the total torque at the shaft 2,and therefore the tension on the rope 5, may surge well above theregulated maximum during transient conditions. The reason for this sharprise can be attributed partially to the time delays in the controlsystem 8. However, it is a teaching of the present invention that theinertias of the motor rotor, the shaft 2, the transmission 3, and thedrum 4 also contribute substantially to the sharp rise in load torqueand must be considered. Specifically, these elements are rotatingrapidly during hoisting, and when the weight 7 is suddenly stopped by animmovable object, the inertias of these revolving elements generateincreased tension on the rope 5, even though the control system 8deenergizes the motor 1.

An indication of the problems encountered with prior art torque sensingand torque control systems is illustrated by the graphs in FIGS. 4a, 4band 4c. When a control system such as that disclosed in the above citedpatent is applied to the hoist-lower control of a Bucyrus-Erie Model280-B excavator, the tension on the hoist rope peaks at a value ofnearly twice the regulated maximum when the dipper impacts against arela tively immovable object. The graph in FIG. 4a plots load torque asa function of time during a digging cycle in which the dipper strikes alarge rock. The desired torque limit is indicated by the dashed line 9.During the initial portion of the cycle, torque is maintained at arelatively low and constant value, however at t=3 seconds impact is madethe load torque rises sharply to a value of approximately 266,000pound-inches, or nearly twice the limit which the hoist control systemis designed to establish. As indicated by the graph in FIG. 4b, which isa plot of hoist motor speed as a function of time, the speed of thehoist motor decelerates rapidly at impact. The load torque begins torise shortly after impact and after peaking eventually reaches thesteady-state torque limit established by the hoist control system. Aplot of the induced torque component as a function of time is shown inFIG. 40. It is immediately apparent from this graph that the hoistcontrol system does, in fact, provide regulation of the induced torquecomponent. Despite this, however, it does not provide adequateregulation of the total torque delivered to the load as evidenced by thegraph in FIG. 4a.

The circuits which are now to be described form part of the controlsystem 8 and constitute an improved means of sensing and indicating theactual torque delivered to the load. The torque feedback signals whichthese circuits generate operate the control system 8 to provide moreeffective control over the torques and forces which occur in the drivesystem elements. It will also become apparent that the improved means ofsensing and indicating torque can be used independently of the controlsystem 8 as a measuring device.

Referring to FIG. 2, a d-c generator designated generally at 10 isconnected in series with a d-c motor designated generally at 11 to forma closed armature circuit loop. The generator 10 includes an armature 12connected in series with a commutating field winding 13 and a seriesfield winding 14. The generator 10 also includes forward field winding15 and reverse field winding 16 which are magnetically coupled to thegenerator armature 12, and which operate to induce a d-c voltage thereinwhen supplied with direct current. The d-c motor 11 includes an armature21 connected in series with a series field winding 22 and a commutatingfield winding 23. The motor 11 also includes a shunt field winding 24connected to a pair of d-c voltage supply terminals 25. The motor 11 andgenerator 10 form a standard Ward-Leonard drive which is commerciallyavailable in many sizes and ratings. When armature current is suppliedto the motor 11 by the generator 10, an induced torque is produced atthe motor'rotor and applied to a shaft 26. The direction of motorarmature current in the loop is determined by current flow in thegenerator field windings l5 and 16, and this direction in turndetermines the direction of the induced torque at the shaft 26. Forpurposes of illustration, when current flows in the generator forwardfield winding 15, loop current flows in the direction indicated by thearrow 27, and the shaft 26 is driven in a forward direction which isindicated by an arrow 27.

The forward field winding 15 and reverse field winding 16 on thegenerator 10 are connected to the outputs of a control system 17. Thecontrol system 17 operates to generate a current through either theforward field winding 15 causing a current to flow in one direction inthe motor-generator loop, or to generate a current to the reverse fieldwinding 16 causing a current to flow in the opposite direction in themotor-generator loop. The magnitude of the field current is determinedin part by the setting of a reference potentiometer 18, the slider ofwhich connects to an input terminal 19 on the control system 17. Onelead of the reference potentiometer 18 is connected to signal ground,and the other lead connects to a d-c voltage supply terminal 20.

The speed of the motor 11 is sensed by a tachometer 28 which is attachedto the shaft 26 and rotated thereby. One lead'of the tachometer 28 isconnected to signal ground, and another lead connects to signal groundthrough a potemtiometer 29. A slider on the potentiometer 29 connects toan input terminal 30 of the controlsystem 17. The tachometer 28generates a speed feedback signal which is proportional in amplitude'tothe speed of-the motor 11. Polarity of the signal indicates thedirection of rotation.

The operation of the servo system thus formed is well known to thoseskilled in the art. A command signal is generated by the referencepotentiometer 18 and is summed in the control system 17 with the speedfeedback signal generated by the tachometer 28 to provide an errorsignal. The-polarity of this error signal determines which of the fieldwindings or 16 is energized, and the magnitude of the error signaldetermines the magnitude of the current flowing in the energized fieldwinding. Thus by selecting the polarity of the d-c voltage applied tothe supply terminal 20 and by selecting the setting of the referencepotentiometer 18, the machinery operator can control the direction andoperating speed of the motor 11. Numerous variations can be made to. thesystem thus far described. For example, instead of using a tachometer tosense motor speed, the voltage across the motor armature 21 and thewinding 22 can'be sensed,.as is done in the aforesaid cited patent. Thisvoltage provides'an indirect indication of motor rotor speed and cantherefore be used as a speed feedback signal for the control system 17.

A torque sensing and indicating circuit incorporating the presentinvention is attached to the servo drive system described above. Thiscircuit operates to generate a torque feedbacksignal through a zenerdiode 33 which is connected to an input terminal 33 on the controlsystem 17. The torque sensing and indicating circuit includes amotorarmature current sensor which is comprised in part of. a firstcoupling resistor 34. The first coupling resistor 34 has one leadconnected to the junction between the motor commutating field winding 23and the motor armature 21, and-its other lead connects to aninvertinginput terminal 35 on an operational amplifier 36. The operationalamplifier 36 has a common terminal 37 which connects to both signalground andto the generator output terminal 31. Consequently, the'voltageestablished across the motor commutating field winding 23 generates acurrent through the first coupling resistor 34 to the amplifierinverting input terminal 35. This current is proportional to the motorarmature current and is, therefore, proportional to the induced torquecomponent at the shaft 26.

The torque sensing and indicating circuit also includes a differentiatorcircuit which connects across the generator output terminals 31 and 32.The differentiator circuit includes a capacitor 39 having one leadconnected to the generator output terminal 32 and another lead connectedthrough a differentiator resistor 40 to generator output terminal 31.The other lead on capacitor 39 also connects through a second couplingresistor 41 to a noninverting input terminal 38 on the operationalamplifier 36. The differentiator circuit operates to sense the voltageapplied to the motor arma- The operational amplifier 36 functions to sumthe induced torque signal received at its inverting input terminal 35and the acceleration torque signal received 'at its noninverting inputterminal 38. It includes a feedback resistor 42 connectedv between theoperational amplifier output terminal 43 and its inverting inputterminal 35. The value of the feedback resistor 42 is chosen inconjunction with the values of the first and second coupling resistors34 and 41 to adjust the gain of the summing circuit. The voltage at theoperational amplifier output terminal 43 is proportional to the sum ofthe induced torque signal and the acceleration torque signal, and itconstitutes the torque feedback signal which is conveyed through thezener diode 33 to the control system 17.

When the motor 11 is operating in theforward direction, current flows inthe motor-generator loop in the direction indicated by the arrow 27; ashereinbefore indicated. Under steady state conditions a voltage is thusestablished across the motor commutating field winding 23 which causes apositive current fiow into the inverting input terminal 35 of theoperational amplifier 36. As a result, the output terminal 43 isdrivento a negative voltage. When the load torque demand on the motor 11becomes excessive (as may occur when motor speed fallsbecause of anincreasing load and the output signal of the tachometer 28 decreases tocause an increased error signal in the control system 17 calling forincreasedmotor speed)'then the negative voltage at the terminal 43reaches the breakdown, or threshold voltage of the zener diode 33; and anegative current is generated to the control system input terminal 33'.This negativecurrent operates to reduce the net error signal generatedby the control system 17 and thereby reduce thecurrent flow in theforward field winding 15. As aresult, both the generator output voltageand motor armature current decrease to slow the speed of the motor 21and reduce the torque demand. The threshold nature of the zener diode 33provides the desired upper limit on the torque output of the motor Whensudden, or abrupt transient speed changes occur, the differentiatorcircuit comes into play. Specifically, whenth'e motor 11 is deceleratedupon encountering a suddenly imposed large load, the voltage across thegenerator output terminals 31 and 32 drops. A negative currentproportional in magnitude to the rate of change of this voltage drop isgenerated by the differentiator circuit and fed to the amplifiernoninverting input terminal 38. This negative current represents anincrease in the total torque at the shaft 26 and is proportional to theacceleration torque produced during the sudden speed change. It is addedto the negative current generated at the amplifier output terminal 43which is attributable to the induced torque component, and it operatesto further reduce the net error signal generated by the control system17. A further reduction in the electrical energy supplied to the motorarmature 21 results. In fact, when large decelerations occur, themagnitude of the acceleration torque component may be large enough toreverse the polarity of the net error signal generated by the controlsystem 17. When this occurs the functions of the generator 10 and motor1] reverse. That is, the motor ll begins to generate electrical power tothe generator 10. The energy thus transferred is derived from thekinetic energy of the driven mechanical system and it is one of theadvantages of the present invention that the control system 17 isoperated by the torque sensing circuit to dissipate this energy aselectrical energy in the Ward- Leonard drive rather than allowing it tobe dissipated as heat in the stretching of ropes or straining of shaftsand gears in the driven mechanical system.

As the above discussion indicates, the torque feedback circuit or FIG. 2generates a torque feedback signal which is comprised of two components:an acceleration torque signal generated by the differentiator circuit;and aninduced torque signal generated by the armature current sensingcircuit. The resulting torque feedback signal at the output terminal 43of the operational amplifier 36 is a true indication of the total torqueon the shaft 26 during both steady-state and transient speed conditions.In fact, a voltmeter indicated at 78 may be connected 'at the amplifieroutput 43 to provide a visual indication of load torque.

A particularly useful application of the present invention to the hoistdrive. system of alarge excavator is shown in FIG. 3, but in thisembodiment of the invention the total torque feedback signal iscomprised of components which when summed together are not necessarilyproportional .to total load torque. Instead, this circuit generates twotorque components: a steadystate induced torque signal which isproportional to the average induced torque delivered to the load; and atransient torque signal, which is representative of the fluctuations inthe total load torque. The total torque feedback generated by thissecond embodiment of the invention is, therefore, comprised of atransient torque feedback signal and a steady-state torque feedbacksignal and the magnitude of the transient torque feedback signal can beadjusted independently of the magnitude of the steady-state torquefeedback signal with the result that the reaction of the hoist drivecontrol system to transient loading can be substantially improved.

Referring to FIG. 3, the hoist drive system includes a motor generatorset comprised of a generator 44, a motor 45, and a control system 46.The hoist drive system may be applied, for example, to a Bucyrus-ErieCompany Model 280-B excavator, in which the ratings of the generator44and motor 45 are as follows:

General Electric Generator Model No. CDS 6482 641 KW at 1,800 RPM 475volts General Electric Motor Model No. MDP 620 750 I-Ip. at 720 RPM 460volts The generator 44 includes an armature 47, a series field winding48 and a commutating field winding 49 connected to form a closed loopwith a motor armature 50,

its series field winding 51 and its commutating field winding 52. Thecontrol system 46 has a first pair of output terminals 53 which connectwith a forward field winding 54 on the generator 44, and a second pairof output terminals 55 which connectwith a reverse field winding 56 onthe generator 44. When the control system 46 generates a current throughthe forward field winding 54, a current is generated in themotorgenerator loop in the direction indicated by the arrow 57. Theresulting torque produced by the hoist motor 45 is coupled to a shaft 58which rotates to hoist or lift the excavator dipper through a mechanicaldrive sys tem that is not shown.

The control system 46 is similar to that disclosed in the above citedU.S. Pat.,No. 3,518,444, and it includes a magnetic amplifier, orsaturable reactor (not shown in the drawings); The saturable reactorincludes control windings that generate feedback signals in the controlsystem, which feedback signals are summed magnetically to provide anerror signal for controlling generator field current at either of theoutput terminals 53 or 55.

One saturable reactor control winding of the control system 46 isindicated in FIG. 3 by the reference numeral 59 and is connected to themotor armature circuit to receive a steady-state induced torque feedbacksignal. One lead of the control winding 59 connects through a zenerdiode 60 to the slider of a torque limit potentiometer 61, theresistance element of which is connected across the motor commutatingfield winding 52 at the terminals 62 and 63. The other lead of thecontrol winding 59 also connects to the terminal 63, which ishereinafter referred to as the reference terminal 63. The steady-stateinduced torque feedback circuit thus established generates a current inthe control winding 59 which is proportional to the voltage across thecommutating field winding 52, and therefore, proportional to the motorarmature current. The setting of the torque limit potentiometer 61 andthe reverse breakdown voltage of the zener diode 60 determines the upperlimit on torque which the control system 46 is to establish. That is,when the steady-state induced torque at the shaft 58 equals or exceedsthis upper limit, the positive voltage generated at the slider of thetorque limit potentiometer 61 exceeds the breakdown voltage of the zenerdiode 60 and current flows through the control winding 59 causing thecontrol system 46 to reduce the current flowing in the generator forwardfield winding 54. Motor armature current is thus reduced and the inducedtorque at the shaft 58 drops. This portion of the circuit of FIG. 3 issimilar to the load current indicating subcircuit described in the abovecited patent.

A second control winding 64 is also coupled to the control system 46 andis operable to generate a transient torque feedback signal thereto. Onelead of the second control winding 64 connects to the cathode of a firstcoupling diode 65, the anodeof which connects to the terminal 62 on themotor commutating field winding 52. The other lead of the second controlwinding 64 connects through a first coupling resistor 66 to the anode ofa second diode 67 and to one lead of a coupling capacitor 68. Thecathode of a second diode 67 connects through a second coupling resistor69 to the terminal 62 on the motor commutating field winding 52. Theother lead of the coupling capacitor 68 connects to one lead of adifferentiator capacitor 70 and through a differentiator resistor]! tothe reference terminal 63. The other lead of the differentiatorcapacitor 70 connects to the cathode of a third diode 72 and through afirst voltage divider resistor 73 to a first generator output terminal74. The generator output terminal 74 connects directly to the referenceterminal 63. The anode of the third diode 72 connects through a secondvoltage divider resistor 75 to a second generator output terminal 76.

The second control winding 64 generates a transient torque feedbacksignal to the control system 46. The transient torque feedback signalincludes a first component which is generated by a differentiatorcircuit comprised of the differentiator capacitor 70, the differentiatorresistor 71, and the voltage divider network formed byfirst and secondvoltage divider resistors 73 and 75. This first component isproportional to the acsteadystate conditions when load torque isconstant,

notransient torque feedback signal is generated. How ever, when a sharprise in torque occurs, the first coupling capacitor 68 provides a lowimpedance path through which large currents flow to the controlwinding64. Asa result, a large transient torque feedback signal isapplied to the control system 46 to reduce the powerapplied to the motor45. I

The first and second coupling diodes 65 and 67 allow current flow inonly one direction through the control winding 64; The transient torquefeedback signal is, therefore, generated only while the shaft 58 isbeing driven in one direction by the motor-generator set. Theorientations of the coupling diodes 65 and 67 are such that thetransient torque feedback signal is effective to limit torque outputonly during the excavator hoist mo- I tion. Asa result, the circuitis'effective to limit the high transient torques which are encounteredduring digging,butdoes not inhibit the torque generated when the dipperis being lowered. Since the transient torque feedback circuitry operatesindependently of the steady state torque circuitry, its gain can be setindependently without affecting the steady-state torque limitestablished by the setting of the torque limit potentiometer 61. Thusthe response time of the control system 46-to sudden changes in loadtorque can be Referring to FIG. 5, the total torque delivered by the rmotor 45 to the shaft 58 is shown plotted as a function of time Thedashed line 77 indicates the torque limit which is established byadjusting the slideron the torquelimit potentiometer 61. It is set tothe same value indicated by the dashed line 9. in. FIG. 4a. The'curverepresents the torque on the shaft 58 as the excavator dipperishoisted in a dig motion. At approximately t==3 seconds, the dipperimpacts a relatively immovable object and a sharp rise in load torqueoccurs. Although the load torque rises above the limit established bythe steady-state torque feedback circuit, the load torque quickly dropsto acceptable levels. The effects of the invented circuit of FIG. 3 areapparent when a comparison is made between the peak load torqueindicated by the curve in FIG. and the peak load indicated by the curvein FIG. 4a. Whereas the peak load torque in the control system withoutthe circuit of FIG. 3 reaches a value which is nearly twice that v. ofthe desired maximum, the peak load torque which occurs when the inventedcircuit is used reaches a value of only 1.49 times the maximum loadtorque.

This constitutes a substantial reduction in thetorque at the shaft 58and a reduction in the resulting forces which are applied .to themechanical drive system.

In each-of the embodiments described a voltage signal derived from themotor driving a load is differentiated to arrive at a signal indicativeof rate of change. in motor speed. This'signal is then utilized as anindication of a torque component due to acceleration or deceleration ofparts of the system. This torque component may be summed with additionaltorque signal components, particularly that indicating theelectromagnetically induced torque applied by the motor. Prior controlsystems have relied solely on an'indication of this induced 5 torquewhen it is desired to utilize a torque feedback signal for control. Thepresent invention teaches measurement of the additional accelerationtorque component. This measurement may be made at the motor by indirectmeans, and is indicative of transient torques in the associatedmechanical system. Hence, there is provided a control over torques, orforces, in a mechanical load, and generating an electrical signalproportional to said acceleration force; and

V arithmetically summing the electrical signals representing inducedforce and acceleration force to provide a signal proportional to thetotal force delivered to the load.

2. A method of generating a signal proportional to the torque deliveredto a load driven by an electric motor, the steps comprising:

indirectly sensing the torque induced by said motor and generating an.electrical signal proportional thereto; indirectly sensing theacceleration torque produced by thev inertias of the motor rotor andload, and generating an electrical signal proportional thereto; and I, Iarithmetically summing the electrical signals representing inducedtorque and acceleration torque to provide a torque signal proportionalto the torque delivered to the load.

3. The method as recited in claim 2 .in which the motor is a d-c motor,the induced torqueis indirectly sensed by measuring motor armaturecurrent, and the acceleration torque is indirectly sensed by measuringthe voltage of motor armature circuit windings and differentiating suchmeasured voltage to obtain a signal proportional to theaccelerationtorque.

4. In a electro-mechanical drive system having an electric motorconnected to a mechanical load and to a control system, a circuit forgenerating a torque feedback signal to the control system, thecombination comprising:

an induced torque sensor connected to the motor and being operable togenerate a signal proportional to the induced torque at the motor rotor;

an. acceleration torque sensor connected to the motor and being operableto generate a signal proportional to the torque developed by theinertias of the motor rotor and the load during speed changes; and

summing means connected to receive the induced torque signal and theacceleration torque signal, said summing means being operable toarithmetically add their magnitudes to provide the torque feedbacksignal.

5. The circuit as recited in claim 4 wherein said motor is a d-c motor,said induced torque sensor connects to measure motor armature currentand generate an induced torque signal to said summing means which isproportional thereto, and said acceleration torque sensor includes adifferentiator which connects to measure voltage applied to the motorarmature circuit windings and which generates an acceleration torquesignal to said summing means which is proportional to the firstderivative of the measured voltage.

6. In an electro-mechanical drive system having a d-c electric motorconnected to a mechanical load and to a control system, a circuit forgenerating torque feedback information to the control system, thecombination comprising:

a steady-state torque feedback circuit connected to of the sensedinduced torqueexceeds a preset maximum; and

a transient torque feedback circuit which includes a differentiatorcircuit connected to sense the voltage applied to the motors stationarywindings and generate an acceleration torque signal proportional to thefirst derivative of that voltage, and a current sensing circuitconnected to sense changes in motor armature current and generate asignal which is summed with the acceleration torque signal andcapacitively coupled to said control system to provide a transienttorque feedback signal thereto.

7. The circuit as recited in claim 6 wherein said transient torquefeedback circuit includes a coupling diode connected to block currentflow in one direction and to thereby inhibit the generation of atransient torque feedback signal of one polarity.

1. A method of generating a signal proportional to the force deliveredto a load driven by an energy conversion machine, the steps comprising:indirectly sensing the force induced by said energy conversion machineand generating an electrical signal proportional to said induced force;indirectly sensing the acceleration force generated by the inertias ofthe energy conversion machine and load, and generating an electricalsignal proportional to said acceleration force; and arithmeticallysumming the electrical signals representing induced force andacceleration force to provide a signal proportional to the total forcedelivered to the load.
 2. A method of generating a signal proportionalto the torque delivered to a load driven by an electric motor, the stepscomprising: indirectly sensing the torque induced by said motor andgenerating an electrical signal proportional thereto; indirectly sensingthe acceleration torque produced by the inertias of the motor rotor andload, and generating an electrical signal proportional thereto; andarithmetically summing the electrical signals representing inducedtorque and acceleration torque to provide a torque signal proportionalto the torque delivered to the load.
 3. The method as recited in claim 2in which the motor is a d-c motor, the induced torque is indirectlysensed by measuring motor armature current, and the acceleration torqueis indirectly sensed by measuring the voltage of motor armature circuitwindings and differentiating such measured voltage to obtain a signalproportional to the acceleration torque.
 4. In a electro-mechanicaldrive system having an electric motor connected to a mechanical load andto a control system, a circuit for generating a torque feedback signalto the control system, the combination comprising: an induced torquesensor connected to the motor and being operable to generate a signalproportional to the induced torque at the motor rotor; an accelerationtorque sensor connected to the motor and being operable to generate asignal proportional to the torque developed by the inertias of the motorrotor and the load during speed changes; and summing means connected toreceive the induced torque signal and the acceleration torque signal,said summing means being operable to arithmetically add their magnitudesto provide the torque feedback signal.
 5. The circuit as recited inclaim 4 wherein said motor is a d-c motor, said induced torque sensorconnects to measure motor armature current and generate an inducedtorque signal to said summing means which is proportional thereto, andsaid acceleration torque sensor includes a differentiator which connectsto measure voltage applied to the motor armature circuit windings andwhich generates an acceleration torque signal to said summing meanswhich is proportional to the first derivative of the measured voltage.6. In an electro-mechanical drive system having a d-c electric motorconnected to a mechanical load and to a control system, a circuit forgenerating torque feedback information to the control system, thecombination comprising: a steady-state torque feedback circuit connectedto indirectly sense the induced torque delivered to the load by themotor and generate a torque feedback signal to said control system whenthe magnitude of the sensed induced torque exceeds a preset maximum; anda transient torque feedback circuit which includes a differentiatorcircuit connected to sense the voltage applied to the motor''sstationary windings and generate an acceleration torque signalproportional to the first derivative of that voltage, and a currentsensing circuit connected to sense changes in motor armature current andgeNerate a signal which is summed with the acceleration torque signaland capacitively coupled to said control system to provide a transienttorque feedback signal thereto.
 7. The circuit as recited in claim 6wherein said transient torque feedback circuit includes a coupling diodeconnected to block current flow in one direction and to thereby inhibitthe generation of a transient torque feedback signal of one polarity.